1. Field of the Invention
The present invention relates to a power supply apparatus provided with a step down switching regulator and a linear regulator, and in particular to control technology for switching the switching regulator and the linear regulator.
2. Description of the Related Art
In recent years, microcomputers for performing digital signal processing are being installed in various electronic devices, such as mobile telephones, PDAs (Personal Digital Assistants), laptop personal computers, and the like. Power supply voltage necessary for driving these microcomputers decreases as semiconductor manufacturing processes become more refined, and there are devices that operate at low voltages of 1.5 volts or below.
Furthermore, lithium-ion batteries or the like are installed as power sources in these electronic devices. The voltage outputted from lithium-ion batteries is approximately 3 to 4 volts, and since electrical power consumption is wasted in supplying the voltage at this rate to the microcomputer, in general, battery voltage is stepped down using a step down type of switching regulator, a series regulator, or the like, and the voltage is made constant, to be supplied to the microcomputer.
With regard to the step down switching regulator, there is a type that uses a diode for rectification (referred to below as a diode rectifier type), and a type that, instead of the diode, uses a synchronous rectifier transistor (referred to below as a synchronous rectification type). The former has an advantage in that high efficiency can be obtained when a load current flowing in a load is low, but since the diode is necessary, in addition to an inductor and an output capacitor at an outer portion of a control circuit, circuit area becomes large. With the latter, when current supplied to the load is small, efficiency is inferior compared to the former, but since the transistor is used instead of the diode, integration is possible inside an LSI, and circuit area including peripheral parts can be miniaturized. With regard to electronic devices such as mobile telephones and the like, in cases in which miniaturization is required, the switching regulator that uses a rectification transistor (referred to below as a synchronous rectification type of switching regulator) is often employed.
Here, consumed electrical current of the microcomputer used in the abovementioned electronic devices varies considerably at operating time and at standby time; only a little current is flowing at standby time, but some amount of current is necessary at operating time.
For example, Patent Documents 1 and 2 disclose a switching regulator for switching a synchronous rectification type and a diode rectifier type, according to the load current.
Patent Document 1: Japanese Patent Application, Laid Open No. 2004-32875
Patent Document 2: Japanese Patent Application, Laid Open No. 2002-252971
FIGS. 11A and 11B show a time waveform of electrical current for a heavy load and a light load of a synchronous rectification type switching regulator. In these figures, IL expresses current flowing in an inductor, Io expresses load current, and time average value of the current IL flowing in the inductor is the load current Io. As shown in FIG. 11A, since, when the load is heavy, the load current Io is large, the current IL flowing in the inductor continues to take a positive value. However, as shown in FIG. 11B, when the load current Io decreases when the load is light, the current IL flowing in the inductor is negative, as in the shaded areas, and the direction of the current IL flowing in the inductor is reversed. As a result, in the synchronous rectification type, when the load is light, the current flows from the inductor via the synchronous rectifier transistor to ground. This current is not supplied to the load, and since it is supplied from an output capacitor, electrical power is needlessly consumed.
In order to solve this problem, technology is known in which the current flowing in the inductor is monitored, and by compulsorily switching OFF the synchronous rectifier transistor when the direction of the current flowing in the inductor is reversed, the current is prevented from flowing to ground. Here, since a duty ratio of a pulse width modulation signal is determined as a ratio of ON time of a switching transistor and ON time of the synchronous rectifier transistor, the synchronous rectifier transistor is compulsorily switched off, and when this ON time becomes short, the ON time of the switching transistor also becomes short.
When the load is light, by the ON time of the switching transistor and the synchronous rectifier transistor being made short, consumed current of the switching regulator decreases; however, since current is consumed in a pulse width modulator or a driver circuit of the switching regulator, there is room for further lowering of consumed current. Here, since, in general, when output current is small, the efficiency of a linear regulator is higher than the efficiency of the switching regulator, the linear regulator and the switching regulator are arranged in parallel, and by switching the two when the load is light, efficiency can be further improved.